No abstract

The Sargasso Sea has long been considered as the only spawning area for Atlantic eels, despite the absence of direct observations. The present study raises a novel scenario, deviating from Schmidt’s dogma, begins with a review of historical and recent observations that were combined to build up a global theory on spawning ecology and migration behavior of Atlantic eels. From this, it is argued that a favorable spawning area could be located eastward of Sargasso Sea at the intersection between the Mid-Atlantic Ridge and the oceanic fronts. Ocean circulation models combined with 3D particle-tracking method confirmed that spawning at this specific area would result in larval distribution fitting the field observation. This study explores the hypothesis that leptocephali are able to swim and orientate to reach their specific growth areas. It proposes a novel framework about spawning ecology, based on orientation, navigation and meeting cues of silver eels to the spawning area. Together this framework may serve as a stepping-stone for solving the long-lasting mystery of eel reproduction which first came out 2,400 years ago and promotes the understanding of oceanic migration and reproduction of marine organisms.

Recruitment of Japanese eels, Anguilla japonica, has declined in recent decades possibly due to both anthropogenic and ocean-atmosphere factors. The potential impact of ocean circulation on the decreasing Japanese eel catches in the western North Pacific was examined based on a three-dimensional particle-tracking method, in which virtual larvae (v-larvae) were programmed to swim horizontally and vertically, in addition to being transported by ocean currents after being released in their North Equatorial Current (NEC) spawning area. Transport patterns varied among years between 1993 and 2013, and dispersion of v-larvae towards East Asia decreased in the last two decades, especially for the western Taiwan and Japan regions. In recent years, instead of entering the Kuroshio and moving towards East Asia as in the 1990s’, more v-larvae tended to enter the southern areas due to the weakening of the NEC and strengthening of subsurface southward flow near the spawning area. Changes in ocean circulation in the western Pacific appear to be caused by the weakening of subtropical and tropical wind stress curl in the past two decades. This suggests that decadal changes in ocean circulation have occurred that affect the larval migration success of the Japanese eel to their recruitment areas.

The possible effect of directional larval swimming on the recruitment success of the Japanese eel, Anguilla japonica, was examined with a three-dimensional particle-tracking ocean circulation model using horizontal northwestward swimming and diel vertical migration (DVM). Four separate experiments included virtual larvae (v-larvae) movement from the spawning area over 290 days (total migration) and 160 days (stage A), from the STCC eddy region in 70 days (stage B), and from the origin of the Kuroshio in 60 days (stage C) to evaluate the effect of directional swimming and DVM compared to simple drifting. Passive or random swimming were not the most effective strategies for larvae dispersing from the spawning area because most v-larvae remained south of 20°N without entering the Kuroshio. Northwestward swimming resulted in wider dispersion and a better chance of successful recruitment, with v-larvae becoming widely distributed in the STCC eddy zone, arriving at the east coast of the Philippines (stage A), escaping the STCC eddy area and reaching the Kuroshio (stage B), and crossing the Kuroshio into the East China Sea shelf (stage C). DVM slightly shortened the migration period due to faster shallow layer ocean currents during nighttime. The NEC transported non-swimming v-larvae westward to the Kuroshio and occasionally northward into the Subtropical Countercurrent (STCC) area where eddies transported v-larvae westward into the Kuroshio, but less than with swimming. Directional swimming increased recruitment success, northwestward swimming was more effective than other directions, and a slower swimming speed was still better than no/random swimming in sensitivity tests. The present study demonstrated a first view of the possibility that Japanese eel larvae might be able to use a strategy of single-direction swimming to increase arrival at their recruitment areas.

The physical and biological roles of mesoscale eddies in Japanese eel larvae dispersal are investigated using a three-dimensional (3D) particle-tracking method, with a focus on the Subtropical Counter Current eddies of the western North Pacific Ocean. Virtual eel larvae (v-larvae) movements depends on the 3D ocean currents and active swimming behavior, including vertical swimming (diel vertical migration), horizontal directional swimming toward settlement habitat, and horizontal swimming toward available food. V-larvae are able to remain in eddies passively due to mesoscale eddy nonlinearity and/or actively due to attraction to rich food supplies. Thus, both physical trapping and biological attraction to food contribute to the retention of v-larvae in eddies. Physical trapping dominates the retention of v-larvae whose swimming speeds are slower than the eddy propagation speed, whereas biological food attraction prevails in the retention of v-larvae swimming faster than eddy propagation. Food availability differs between warm (anti-cyclonic) and cold (cyclonic) eddies, with the latter providing a richer food supply. Fish larvae that are retained for longer durations in cold eddies (shorter durations in warm eddies) are able to obtain more food and potentially grow faster, which enhances survival rates.

An atmospheric component of the Philippine–Taiwan Oscillations (PTOa) is used to examine its potential connection with tropical cyclone (TC) frequency in the western North Pacific in the period of 1979–2014. During positive PTOa years, more TCs are observed in the tropical western Pacific south of 18 °N where cyclonic wind anomaly appears. On the other hand, anti-cyclonic wind anomaly appears in the subtropics north of 18 °N and is associated with less TC formation there. The opposite wind vorticities in tropics/subtropics and associated variability in TC frequency reverse in negative PTOa phase. Besides, the negative contribution of ocean heat content suggests the relative importance of wind vorticity according to the oceanic cyclone genesis potential index. The PTOa provides a more direct explanation of the TC activity compared to other remotely linked phenomena at least for the past 36 years (1979–2014). Therefore, PTOa can potentially serve as a promising index for diagnosing or forecasting TC activity in the western North Pacific Ocean.

No abstract

Japanese eel (Anguilla japonica) recruitment to Japan was very low during the early 2017−2018 recruitment season when most glass eels are usually caught, but catches increased in the late recruitment season when recruitment usually decreases. Concurrently, the Kuroshio meander south of Japan had formed again after the previous event ended in 2005. The role of the large meander and ocean circulation features such as the North Equatorial Current (NEC) in the unusual 2017−2018 Japanese eel recruitment timing-pattern was investigated using a three-dimensional particle tracking model that simulated swimming behaviors of virtual larvae (v-larvae) in addition to their drift in ocean currents. Four recruitment seasons were selected for when the Kuroshio large meander was present (2004−2005, 2017−2018) or absent (2009−2010, 2015−2016), and when NEC was shifted north (2004−2005, 2015−2016) or south (2009−2010, 2017−2018). The simulated recruitment timing-patterns were similar to the actual recent-year recruitment, with no early recruitment period v-larvae arrival to southern Japan and increased late period recruitment occurring. Rather than being related to the presence of the Kuroshio large meander, the late arrival appeared to be caused by a southward shifted, weak North Equatorial Current near the spawning area, a longer Subtropical Countercurrent eddy region retention time, and a weak Kuroshio during the early migration and recruitment period of those years. In the late recruitment period, the Kuroshio was stronger than other selected years near the East China Sea and south of Japan and v-larvae were transported more rapidly. The Kuroshio large meander may influence local eel recruitment in Japan, and the recirculation formed by the large meander could potentially enhance recruitment to the Tokai region. Oriented (northwestward) swimming v-larvae were less affected by the large meander, and showed higher recruitment success than those using along-current swimming. Although the Kuroshio large meander did not seem to be responsible for the unusual recruitment pattern in 2018, how Japanese eel larvae and glass eels actually cross out of the Kuroshio and reach coastal waters in Japan remains to be explored.